I gotta agree with moskus about mixing in CEP, but I guess it would really depend on how you like to work.
I have big problems getting more than about 10 tracks to play properly in CEP, it seems that no matter how I try to set up the buffers in CEP, my HD just runs like crazy with more than 10 or so tracks playing, Sonar on the other hand, I have set up to run nice and smooth..
CEP also seems to get choked up really fast with real-time plugins, you can, and pretty much have to "lock" the effects to the tracks, but this makes the project loading times take almost as long as putting a different reel of tape onto the old reel to reel!!!
Plus the lack of a proper aux send/return setup in CEP cramps the way I like to record, on my old P-III 600 Sonar REALLY outshines CEP in just about all areas of muti-track performance, but as an editing, file conversion, and home mastering program, CEP rules...